CMG, VMG, DMG, What are they?

[Bill Ebling]

I understand if you have the following sensors feeding into your instrument bus: Compass, Wind direction and speed, Knot meter, and geographical position (GPS or Loran), you have enough information for an instrument package to compute among others the following:Velocity made good...VMGCourse made good...CMGDistance made good...DMG.I understand all the other possible metrics that can be computed except these. With the metrics VMG, CMG and DMG, what exactly are you "making good" to? (A target way point???) How are these metrics computed?How would a helmsman/navigator use them? Thanks. Bill EblingYesterday's Dream1985 H31' Hull #31395Crab Alley Bay, Kent Island, MD

 

[Miles]

To a target waypoint...

Generally the "made good" measurements refer to progress toward a specific waypoint. VMG for example is basically a vector sum of your tacks toward a waypoint. Let's say the waypoint is dead up wind so you can't sail straight toward it. You can tack back and forth at maybe 40 degrees until you reach it at say 6 knots. Your "velocity made good" is the speed at which you are approaching the waypoint which might be something like 4 knots, the remaining 2 knots are wasted by going in the wrong direction on each tack. This figure is usefull because each boat has an optimal angle to sail up or downwind, the higher you point the slower you go but the less speed you "waste" going in the wrong direction. Many times when we talk about VMG we're refering to an imaginary waypoint that is dead up wind (for the sake of comparing different boats performance specs.) but it can also apply to a real waypoint for navigation purposes. CMG and DMG are affected by things like currents. Say you're sailing 180 degrees (due south) but there is a 2 knot current from the west, your course made good won't be 180 degrees because you'll be set off to the east.A good GPS (even a cheap GPS) can calulate these figures and display them much faster than you can work them out on paper. Ahhh, the wonders of technology!

 

[Bill Ebling]

So you must have a position sensor...

...to compute VMG? In the WM catalog the raytheon ST60 "Close-hauled

 

[Miles]

Hmmm...

Without an active waypoint it probably shows you VMG dead upwind. Since it has sensors for the apparent wind speed and angle as well as your boat speed it can compute a good approximation of the true wind speed and angle and then figure out what component of your boat speed is dead upwind. I don't know if it switches over to using a waypoint if one becomes active on the bus but I suppose it might. A GPS based calculation of VMG to a waypoint would be a lot more accurate and probably more relevant for navigation than a dead upwind VMG. I think COG and SOG are the same as CMG and SMG. The link below has all sorts of information on NMEA sentences and what data each contains. It's pretty "geeky" but might be interesting if you want to understand how all that stuff works. Have fun!

 

[Les Andersen]

Navigation Terms

Bill,Try this link. It has just about everything you ever wanted to know about navigation (and alot you didn't). The acronyms you asked about are there you just have to look them up.Good reading.Les Andersens/v Mutual Fun